Good explanation from Big Dog. Further explanation.
Combinability of LLC's, Corporations, Partnerships and Sole Proprietorships is generally based on one of the officers of the Corporation owning at least 51%, with that same officer (if wanting to combine with another entity), having at least 51% ownership of the entity it is wanting to combine with, if it is a partnership wanting to combine with an LLC, or Corporation, then one of the Partners must also have at least 51% ownership of the partnership as well as at least 51% ownership of the other entity or entities they would like to combine with. If it is a sole proprietorship (100% ownership), the sole proprietor who wants to combine with another entity must have at least 51% ownership of the entity it wants to combine with. With sole proprietorships wanting to combine with other entities it sometimes can be an issue for underwriters for various reasons. But it never hurts to ask!
This applies to general liability, automobile, umbrella's and any other type of liability policy desired.
This is provided by me a person with 13 years commercial underwriting experience. Hope this helps further clarify.